The present invention relates to an electric motor that detects the rotational position of a rotor by a rotation sensor and incorporates a control circuit for controlling a drive current based on the detected rotational position.
A typical vehicle engine includes a variable valve timing device that varies the valve timing in accordance with the operating state of the engine. The variable valve timing device changes the rotation phase of a camshaft relative to a crankshaft of the engine. The variable valve timing device uses, for example, a stepping motor as a drive source (see Japanese Laid-Open Patent Publication No. 11-153008).
In general, it is difficult to control the rotation phase with high accuracy using a stepping motor and measures for preventing loss of synchronism as disclosed in the foregoing patent document are also necessary. Therefore, a variable valve timing device that uses a brushless motor instead of the stepping motor has been proposed to perform highly accurate control.
FIG. 4 shows a conventional brushless motor 51. As shown in FIG. 4, the brushless motor 51 has a substantially cup-shaped housing 52. The housing 52 accommodates a stator 53 and a magnet-embedded type rotor 54. The opening of the housing 52 is closed by an aluminum frame 55.
The stator 53 is cylindrical and includes a stator core 57, which has teeth, and a coil 59 wound about the teeth. The rotor 54 is rotatably accommodated inward of the stator 53. The rotor 54 includes a rotor core 60, which is formed by laminating disk-like core sheets. A rotary shaft 62 is inserted in a through-hole 61 formed at the center of the rotor core 60. The rotary shaft 62 is rotatably supported by bearings 63, each of which is provided in one of the housing 52 and the aluminum frame 55.
A sensor magnet 65 for detecting the position is secured to the end portion of the rotor 54. A rotation sensor 66 is located opposite to the sensor magnet 65 in the housing 52. The rotation sensor 66 detects the rotational position of the rotor 54 and sends a detection signal to a control circuit 68 provided outside the motor. The control circuit 68 supplies a drive current to the coil 59 of the stator 53 in accordance with the rotational position of the rotor 54. A rotating magnetic field is produced in the stator 53 as the drive current is supplied to the coil 59. The rotating magnetic field rotates the rotor 54.
However, since the control circuit 68 is separately provided according to the conventional brushless motor 51, the rotation sensor 66 inside the motor must be connected to the control circuit 68 with a lead wire, or the like. Thus, the cost is increased by providing wiring material, such as a lead wire, and securing member for securing the wiring material. It is also required to provide space for arranging the lead wire.